Project Summary/Abstract The urea cycle is the major pathway for detoxification of ammonia in mammals. Arginase 1 deficiency, a distal urea cycle disorder, results in hyperargininemia. In humans, deficiency of this enzyme is characterized clinically by progressive mental impairment, spasticity, and growth retardation; periodic episodes of hyperammonemia are uncommon unlike the other urea cycle disorders where this is a much more frequent occurrence. In recent experiments, substantial anatomical, ultrastructural and electrophysiological differences between knockout animals and wild type controls were found. Interestingly, arginase deficient young mice have decreased dendritic arborization and decreased synaptic density in layer V cortical neurons. However, after adeno-associated virus (AAV)-based gene therapy was administered on postnatal day 2, these abnormalities, when examined at two weeks of age, were rescued after nearly normalizing plasma arginine during this critical period of central nervous system development. The discovery of these measurable differences at the neuron and synapse level, and their prevention with gene therapy, have begun to elucidate the previously unknown functional neurological abnormalities that are present in arginase deficiency and have implications as new therapies are sought. This limited proposal will examine this further under the hypothesis that control of arginine and its metabolites are critical during early central nervous system development and if this can be achieved, will lead to long-term prevention of neuronal dysfunction. Preliminary data: Our research group has: 1) constructed and characterized the arginase 1 knockout mouse; 2) demonstrated long-term survival and rescue with recombinant AAV; 3) demonstrated that only low-level ureagenesis is necessary for long-term survival; 4) shown, using an array of behavioral tests, that treated arginase knockout animals lack behavioral abnormalities and there is no difference in learning when compared to littermates; 5) shown that peripheral metabolism can result in control of circulating plasma arginine; and 6) shown that loss of arginase gene expression results in abnormalities of intrinsic excitability, synapse density, and the dendritic arbor of neurons. In this proposal, our existing studies will be expanded upon by testing the hypothesis that control of arginine and its metabolites during early central nervous system development results in long-lasting changes at the neuron and dendrite level and further will explore myelination as an additional source of dysregulation as suggested by microarray analysis. Successful completion of these studies will provide further evidence supporting neonatal gene therapy for arginase deficiency by demonstrating that rigorous control of plasma arginine during postnatal brain maturation, not attainable by current methods, is essential for the prevention of the neuropathology associated with hyperargininemia.